First of all, thank you for your kind words about the book. I am glad you enjoyed it.
There is a grammar and several hundred words.
Which doesn't save me from having to invent new words when I find there is something in a spell that I want to say and hadn't anticipated. When that happens, I try to find a related word already included in the vocabulary and use a variation of that. Otherwise, I usually mangle a word out of one of the Welsh or Irish dictionaries I keep on my shelf.
There are far, far more words in the language than I will ever be able to use in the story. However, in the second book,
A Dark Sacrifice, there is a glossary that defines the different kinds of magic, and gives the meanings of all the names that are based on Niadhélen, as well as units of time.
Also, there is much more in this thread
http://www.sffchronicles.co.uk/forum/42316-maps-and-some-other-things-you-wont-see.html which contains many things I originally put into the glossary but my editor removed. This includes translations of all the spells, and also two maps.